Exergy Analysis in Buildings A complementary approach to energy analysis Exergy Analysis in Buildings A complementary approach to energy analysis Marco Molinari Licenciate thesis KTH-Stockholm, Sweden November 2009 KTH The Royal Institute of Technology School of Architecture and the Built Environment Division of Building Technology KTH, Royal Institute of Technology School of Architecture and the Built Environment Department of Civil and Architectural Engineering Division of Building Technology Brinelsvägen 34 SE-10044 Stockholm Sweden Printed in Sweden by Universitetservice US AB ISBN-978-91-7415-519-8 ISSN-1651-5536 ISRN-KTH-BYT/R09/203-SE © 2009 Marco Molinari Act practically, think theoretically, live idealistic Abstract Though mandatory to be pursued, improved energy efficiency is not the only target to reach. The quality of energy has to be assessed as well. Most of the overall energy use in residential building is for low temperature heat, i.e. temperatures relatively close to the outdoor conditions. From a thermodynamic point of view, this is a degraded form of energy with low potential to be converted into work. On the other hand energy demand is mostly met with high quality energy, such as electricity and natural gas. There is a mismatch between supply and demand, which is not clearly shown by the sole energy analysis. Target of this thesis is to analyze the energy use in buildings from the point of view of its quality, to provide effective theoretical and calculation tools to investigate this mismatch, to assess its magnitudo and to propose improvements aiming at a more rational use of the energy. The idea behind the quality is clarified with the concept of exergy. The potential for improvement in space heating is shown. In no heating system the overall exergy efficiency is above 20%, with fossil fuels. Using direct electricity heating results in exergy efficiency below 7%. Most of the household appliances processes have low-exergy factors but still are supplied with electricity. This results in poor exergy efficiencies and large exergy losses. Systems are poorly performing because little consideration is explicitly given to energy quality. Policies to lower the energy demand, though vital as first step towards an improved use of energy, should not neglect the exergy content. The problem is then shifted to find suitable supplies. Electricity can be exploited with low exergy losses with high-COP heat pumps. Use of fossil fuels for heating purposes should be avoided. District heating from cogeneration and geothermal proves to be a suitable solution at the building level. The issues connected to its exploitation forces to shift the boundary layers of the analysis from the building level to the community level. A rational use of energy should address the community level. The system boundaries have to be enlarged to a dimension where both the energy conversion and use take place with reduced energy transportation losses. This is a cost-effective way to avoid the waste of the exergy potential of the sources with exergy cascade and to make it possible the integration of with renewable sources. Exergy efficiency of the buildings is a prerequisite for a better of energy in this field. Keywords Exergy; Energy; Building systems performance; Built environment. i ii Aknowledgements This thesis is the result of my work at the Division of Building Technology at KTH – The Royal Institute of Technology in the past two years. Two years full of experiences and people. It would be too demanding to remember all the persons that directly or indirectly took part in this work, from a professional and –more important- personal point of view. Also, it is reductive to summarize in one name what persons signify for us. In this context I would like to acknowledge the financial support of the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (FORMAS) and of the European Science Foundation COST C24. I would like to express my gratitude to Prof. Jóhannesson for this chance and, moreover, for the witty and inspiring night talks. My thankfulness is also for Dr. Schmidt, Dietrich, Prof. Björk, Prof. Zecchin and Prof. de Carli. A special thought is for my family, always supporting me with sense of humor. I would like also to remember here my friends, the evergreen ones in Venice, the ones that made colorful the life in Stockholm and the ones from those two wonderful years in Kassel who are now quite everywhere. They all, though physically distant sometimes, are always close. To this people I dedicate this thesis, because, directly or indirectly, they are all here. iii List of enclosed papers I Molinari M., Energy management in buildings: matching supply and demand by means of exergy. To be submitted. II Molinari M., Exergy efficient space heating systems: analysis of different solutions. To be submitted. III Molinari M., (2008). An exergetic analysis and potential for improving the rational energy use in dwellings. In: Proceedings of the 8th Symposium on Building Physics in the Nordic Countries, Copenhagen, Vol. 2, pp.613-620. IV Molinari M., (2009). A pressure and thermal exergy analysis of a waterborne and an airborne system In: Proceedings of the 15th „Building Services, Mechanical and Building Industry Days” International Conference, Debrecen, Building energy, pp73-80. V Molinari M., (2009). SEPE: an Excel calculation tool for exergy-based optimizations. In: ECBCS Annex 49 newsletter 6 iv v Table of contents Abstract .......................................................................................................................... i Keywords ....................................................................................................................... i Aknowledgements ...................................................................................................... iii List of enclosed papers .............................................................................................. iv Table of contents ........................................................................................................ vi 1. Introduction ......................................................................................................... 1 World energy use ..................................................................................................... 1 Buildings energy use................................................................................................ 1 2. Exergy method .................................................................................................... 4 Assessing the potential: the theory of exergy...................................................... 4 Evaluating the performance: the exergy of the different forms of energy and the exergy efficiency ................................................................................................ 6 Exergy analysis in buildings ................................................................................... 8 3. ANNEX 49 & COST Projects ....................................................................... 10 4. Previous studies................................................................................................. 12 Theoretical issues in exergy ................................................................................. 12 Exergy in buildings ................................................................................................ 12 Exergy in geothermal, district heating & heat pumps ..................................... 13 Exergy in storage systems .................................................................................... 14 Exergy and sustainability ...................................................................................... 15 5. Exergy analysis tools: SEPE ........................................................................... 16 Program logic ......................................................................................................... 18 Models description ................................................................................................ 19 6. Papers ................................................................................................................. 21 Energy management in buildings: matching supply and demand by means of exergy.................................................................................................................. 21 Exergy efficient space heating systems: analysis of different solutions ........ 22 vi An exergetic analysis and potential for improving the rational energy use in dwellings. (Proceedings of the 8th Symposium on Building Physics in the Nordic Countries, Copenhagen, 2008) .............................................................. 22 A pressure and thermal exergy analysis of a waterborne and an airborne system. (Proceedings of the 15th „Building Services, Mechanical and Building Industry Days” International Conference, Debrecen 2009) .......... 23 SEPE: an excel calculation tool for exergy-based optimizations. (ECBCS Annex 49 newsletter 6, September 2009) .......................................................... 23 7. Results ................................................................................................................. 24 8. Discussion and conclusions ............................................................................ 26 9. Bibliography ......................................................................................................